1. The Field of the Invention
The present invention relates to the field of electronic communication. In particular, the present invention relates to systems and methods for detecting tampering of a computer system by calculating a boot signature, the boot signature being calculated using a sequence of signals generated during boot time within the computer system.
2. The Prior State of the Art
Electronic information is currently available in a variety of forms. Electronic information that is designed for presentation to a user will typically be in a form such that it may be rendered on a user interface device. For example, electronic information such as video, image, text, font and layout data may be displayed on a monitor thereby engaging a user's sense of sight. Electronic information such as audio may be sounded with a speaker thereby engaging a user's sense of hearing. In the future, with the development of appropriate user interface devices and standards, electronic information may represent data that would engage a user's sense of touch, taste, and smell as well. Electronic information that is designed for presentation to a user will be referred to in this description and in the claims as “presentable content” regardless of the format of the presentable content and regardless of whether standards and user interface devices for the presentable content are currently developed.
There may be many sources of presentable content. Remote sources might include, for example, radio broadcasters, television broadcasters, and server computer systems. Local source might include, for example, a local memory or a local server computer system. These sources will be referred to in this description and in the claims as “content sources” regardless of the particular source of the presentable content and regardless of whether the source is remote or local.
It may often be desirable to limit access to presentable content. For example, a television broadcaster may design that access to their channel be limited to only those users who have properly subscribed to that channel. A television broadcaster may also allow access on a program-by-program basis as in pay-per-view television. A content source such as a Web page provider may also desire to limit access to premium Web pages upon the payment of certain consideration. In these cases, the content source may typically encrypt the presentable content before transmission to the user. A content source such as a broadcast network or even a local memory device may store the presentable content in an already encrypted state thereby foregoing the need to encrypt the content again.
In order for an authorized user to be able to access encrypted presentable content on a user interface device, the corresponding system associated with the user interface device must have access to a service that determines that the user is authorized, and that decrypts the encrypted presentable content when it determines that the user is authorized. A set top box or a component integrated with a television monitor may be suitable devices for performing such a service for encrypted television programming. Devices that perform this service will be referred to in this description and in the claims as a “conditional access device.”
Conventional conditional access devices typically include a decrypter that has access to encrypted presentable content requested by a user. For example, in television broadcasting, a user may control a tuner which tunes to one of the many channels that the conditional access device receives. The tuned channel is then demodulated and presented to the decrypter.
The decrypter is designed to respond to an appropriate key word or other authenticating string. Typically, unless provided with the key word, decrypters will either not pass any signal through to the user interface devices or will pass only the encrypted content through to the user interface devices. In either case, the user does not have access to the presentable content. However, once the appropriate key word is provided to the decrypter, the decrypter is activated so as to decrypt the encrypted presentable content and pass the presentable content to the user interface devices for presentation to the user. The decrypter is active indefinitely or until the happening of a certain event. Such events may include the receipt of another key word that deactivates the decrypter, the end of a certain presentable content segment, or the passage of a specified period of time.
There are a variety of ways that the key word can be provided to the decrypter when the user is authorized. For example, in the Digital Video Broadcast (DVB) standard, an Entitlement Control Message (ECM) is provided “in band” within the digital video broadcast. “In band” means in the same channel or frequency spectrum as the corresponding presentable content. The entitlement control message is processed by the local Central Processing Unit (CPU). If the entitlement control message indicates authority to access the digital video broadcast, the CPU causes a control word to be sent to the decrypter. This control word may be the key word that activates the decrypter or may be a word that enables the decrypter to load the key word from memory.
It may be possible to tamper with the content of local memory in order to obtain access to a presentable content segment even though the user is not so authorized. Typically, this might involve altering the operating system so that the key word is always provided to the decrypter even if the user is not authorized. This frustrates the purpose for providing a conditional access provider and allows access to presentable content under inappropriate circumstances. Therefore, what are desired are systems and methods for preventing users from tampering with computer systems so as to, for example, gain unauthorized access to presentable content.